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Mirror alignment and focus of point-focus solar concentrators

Description: Distributed point-focusing solar concentrators are being developed for dish-Stirling systems and other applications. Many of these concentrators make use of faceted mirrors that have to be accurately aligned. Some of the solar concentrator designs use stretched-membrane facets that also require focusing. Accurate mirror alignment and focus of faceted solar concentrators have two benefits. First, the concentration ratio of the concentrator/receiver (collector) system is improved with accurate alignment and focus. The receiver aperture diameter can therefore be smaller, thereby reducing thermal losses from the receiver and improving the overall efficiency of the collector. Second, and perhaps more importantly, flux intensities on the receiver can be sensitive to facet alignment and focus. In this paper, the theory and practical application of an alignment and focusing technique are presented. In the technique, light from an artificial source is reflected from the concentrator`s facets to a target. From basic geometric principles, the shape and location of the reflected light on the target can be predicted. Alignment is accomplished by adjusting the facets aim so that the reflected image falls on the predetermined location. To focus a stretched-membrane facet, the reflected image size is adjusted to match that of the target. The governing equations used to draw the alignment targets are developed and the practical application of the technique to the alignment and focus of the Cummins Power Generation, Inc. CPG-460 are presented. Alignment uncertainty associated with this technique on the CPG-460 is also discussed.
Date: November 1, 1994
Creator: Diver, R. B.
Partner: UNT Libraries Government Documents Department

Testing and evaluation of large-area heliostats for solar thermal applications

Description: Two heliostats representing the state-of-the-art in glass-metal designs for central receiver (and photovoltaic tracking) applications were tested and evaluated at the National Solar Thermal Test Facility in Albuquerque, New Mexico from 1986 to 1992. These heliostats have collection areas of 148 and 200 m{sup 2} and represent low-cost designs for heliostats that employ glass-metal mirrors. The evaluation encompassed the performance and operational characteristics of the heliostats, and examined heliostat beam quality, the effect of elevated winds on beam quality, heliostat drives and controls, mirror module reflectance and durability, and the overall operational and maintenance characteristics of the two heliostats. A comprehensive presentation of the results of these and other tests is presented. The results are prefaced by a review of the development (in the United States) of heliostat technology.
Date: February 1, 1993
Creator: Strachan, J. W. & Houser, R. M.
Partner: UNT Libraries Government Documents Department

Development of a 2f optical performance measurement system

Description: Commercialization of faceted stretched-membrane concentrators has created the need for quality control measurements for facet optical performance. High-volume production scenarios require a test method that is quick, can be performed on the production floor, and does not require laboratory-grade equipment. At Sandia, we are developing an optical test method that can be done in a relatively short period of time, requires little space, uses ``off the shelf`` test equipment, and provides a quantitative measure adequate to address quality control requirements. The test method is based on common ray trace calculations for targets and images at the radius of curvature for spherical and parabolic (f/D > 3) concentrators. The initial development work was done using a monochrome television camera and the beam characterization system. Twenty-four 3-meter facets built for a faceted stretched-membrane dish were characterized using this system. Other smaller facets have also been tested with the system to help establish a correlation with this method and other optical characterization tests. Current work involves adapting a color camera to the system and developing custom software to reduce analysis time and make the system viable for the production floor. This paper discusses the development of the monochrome system, reviews the results of testing, and presents plans for the development of a color system.
Date: December 1, 1993
Creator: Grossman, J. W.
Partner: UNT Libraries Government Documents Department

Improved techniques for manufacturing the Alpha Solarco concentrating photovoltaic system. Final subcontract report, 9 January 1991--14 April 1991

Description: This report describes the overall procedure involved in the manufacture of photovoltaic cells and modules; identified and describes potential cell and module manufacturing processes that can lead to improved performance, reduced manufacturing costs and significantly increased production, and notes the long-range potential benefits of these improvements; identifies and describes any problems that may impede the achievement of the long-range benefits; and identifies and describes the approaches for solving the identified implementation problems, including time and cost estimates.
Date: November 1, 1991
Creator: Schmidt, E.
Partner: UNT Libraries Government Documents Department

Advanced reflector materials for solar concentrators

Description: This paper describes the research and development program at the U.S. National Renewable Energy Laboratory (NREL) in advanced reflector materials for solar concentrators. NREL's research thrust is to develop solar reflector materials that maintain high specular reflectance for extended lifetimes under outdoor service conditions and whose cost is significantly lower than existing products. Much of this work has been in collaboration with private-sector companies that have extensive expertise in vacuum-coating and polymer-film technologies. Significant progress and other promising developments will be discussed. These are expected to lead to additional improvements needed to commercialize solar thermal concentration systems and make them economically attractive to the solar manufacturing industry. To explicitly demonstrate the optical durability of candidate reflector materials in real-world service conditions, a network of instrumented outdoor exposure sites has been activated.
Date: October 1, 1994
Creator: Jorgensen, G.; Williams, T. & Wendelin, T.
Partner: UNT Libraries Government Documents Department

Photovoltaic Manufacturing Technology report, Phase 1. Final technical report, 9 January 1991--14 April 1991

Description: This report describes subcontracted research by Spectrolab, Inc., to address tasks outlined in the National Renewable Energy Laboratory's (NREL) Letter of solicitation RC-0-10057. These tasks include the potential of making photovoltaics (PV) a more affordable energy source, as set forth in the goal of the PVMaT project. Spectrolab believes that the DOE cost goals can be met using three different types of cells: (1) silicon concentrator cells, (2) high efficiency GaAs concentrator cells, and (3) mechanically stacked multijunction cells.
Date: October 1, 1992
Creator: Mason, A. V. & Lillington, D. R.
Partner: UNT Libraries Government Documents Department

Solar collector manufacturing activity, 1992

Description: This report presents data provided by US-based manufacturers and importers of solar collectors. Summary data on solar thermal collector shipments are presented for the years 1974 through 1992. Summary data on photovoltaic cell and module shipments are presented for the years 1982 through 1992. Detailed information for solar thermal collectors and photovoltaic cells and modules are presented for 1992. Appendix A describes the survey methodology. Appendix B contains the 1992 survey forms and instructions. Appendices C and D list the companies that responded to the 1992 surveys and granted permission for their names and addresses to appear in the report. Appendix E provides selected tables from this report with data shown in the International System of Units (SI) metric units. Appendix F provides an estimate of installed capacity and energy production from solar collectors for 1992.
Date: November 9, 1993
Partner: UNT Libraries Government Documents Department

Reflective coatings for solar applications

Description: Many applications of solar energy require large mirrors to provide high levels of concentrated sunlight. The success of such conversion systems hinges on the optical durability and economic viability of the reflector materials. A major effort at the National Renewable Energy Laboratory (NREL) has been to improve the existing reflector materials technology and to identify candidates that retain optical performance and durability criteria and offer potential for reduced cost. To attain the goals, it is desirable to maintain and increase the involvement of industrial organizations in reflective materials R&D related to the conversion of solar resources to useful energy. Toward this end, NREL has recently initiated several collaborative efforts with industry to develop advanced reflector materials.
Date: May 1, 1993
Creator: Jorgensen, G.
Partner: UNT Libraries Government Documents Department

Status of photovoltaic concentrator modules and systems

Description: Several leading line- and point-focus photovoltaic concentrator system development programs are reviewed, including those by ENTECH, SEA Corporation, AMONIX, and Alpha Solarco. Concentrating collectors and trackers are gaining maturity and reaching product status as designs are made more manufacturable and reliable. Utilities are starting to take notice of this emerging technology, and several privately-funded utility installations are underway. Several advantages are offered by concentrators, including low system and capital cost and rapid production ramp-up. These are discussed along with issues generally raised concerning concentrator technology.
Date: April 1, 1994
Creator: Maish, A. B.
Partner: UNT Libraries Government Documents Department

Felt-metal-wick heat-pipe solar receiver

Description: Reflux heat-pipe receivers have been identified as a desirable interface to couple a Stirling-cycle engine with a parabolic dish solar concentrator. The reflux receiver provides power nearly isothermally to the engine heater heads while decoupling the heater head design from the solar absorber surface design. The independent design of the receiver and engine heater head leads to higher system efficiency. Heat pipe reflux receivers have been demonstrated at approximately 65 kW{sub t} power throughput. Several 25 to 30-kW{sub e} Stirling-cycle engines are under development, and will soon be incorporated in commercial dish-Stirling systems. These engines will require reflux receivers with power throughput limits reaching 90-kW{sub t}. The extension of heat pipe technology from 60 kW{sub t} to 100 kW{sub t} is not trivial. Current heat pipe wick technology is pushed to its limits. It is necessary to develop and test advanced wick structure technologies to perform this task. Sandia has developed and begun testing a Bekaert Corporation felt metal wick structure fabricated by Porous Metal Products Inc. This wick is about 95% porous, and has liquid permeability a factor of 2 to 8 times higher than conventional technologies for a given maximum pore radius. The wick has been successfully demonstrated in a bench-scale heat pipe, and a full-scale on-sun receiver has been fabricated. This report details the wick design, characterization and installation into a heat pipe receiver, and the results of the bench-scale tests are presented. The wick performance is modeled, and the model results are compared to test results.
Date: December 31, 1994
Creator: Andraka, C. E.; Adkins, D. R.; Moss, T. A.; Cole, H. M. & Andreas, N. H.
Partner: UNT Libraries Government Documents Department

Comparison of predicted optical performance with measured results for dish concentrators

Description: Several optical design tools have been developed at the National Renewable Energy Laboratory (NREL) during the past two years. These have been used extensively both in-house and by industry to analyze dish concentrator systems and to optimize performance of such designs. The first program, OPTDSH, models single-element dish concentrators. The second code, ODMF, allows multifacet dish arrays to be modeled. The accuracy of performance simulations by these programs has been established by comparing predicted results with measured on-sun data. ODMF evolved from NREL`s High-Flux Solar Furnace (HFSF) design tool, SOLFUR, and in fact is a special case of SOLFUR in which the primary facet array is ``on sun.`` Consequently, confirmation of the accuracy of SOLFUR would verify the results from ODMF as well. Furthermore, because OPTDSH can be viewed as a single-facet case of ODMF, determination of the precision of SOLFUR/ODMF would also substantiate OPTDSH. Thus, the approach to verifying the correctness of all three codes was to compare flux patterns as predicted by SOLFUR with those actually measured at NREL`s HFSF. Measured vs. calculated data have been compared on the basis of flux distribution (in terms of contour plots) and peak flux for both single-facet and multiple-facet cases. Agreement in measured vs. predicted peak flux values has been obtained within the uncertainty associated with the measurement/calibration process. Excellent agreement has also been demonstrated by comparing contour maps of measured vs. computed flux levels. 7 refs.
Date: April 1, 1991
Creator: Jorgensen, G.
Partner: UNT Libraries Government Documents Department

Photovoltaic solar concentrator module

Description: This invention consists of a planar photovoltaic concentrator module for producing an electrical signal from incident solar radiation which includes an electrically insulating housing having a front wall, an opposing back wall and a hollow interior. A solar cell having electrical terminals is positioned within the interior of the housing. A planar conductor is connected with a terminal of the solar cell of the same polarity. A lens forming the front wall of the housing is operable to direct solar radiation incident to the lens into the interior of the housing. A refractive optical element in contact with the solar cell and facing the lens receives the solar radiation directed into the interior of the housing by the lens and directs the solar radiation to the solar cell to cause the solar cell to generate an electrical signal. An electrically conductive planar member is positioned in the housing to rest on the housing back wall in supporting relation with the solar cell terminal of opposite polarity. The planar member is operable to dissipate heat radiated by the solar cell as the solar cell generates an electrical signal and further forms a solar cell conductor connected with the solar cell terminal to permit the electrical signal generated by the solar cell to be measured between the planar member and the conductor.
Date: May 16, 1991
Creator: Chiang, C. J.
Partner: UNT Libraries Government Documents Department

Cost effective manufacturing of the SEA 10X concentrator array. Final subcontract report, 9 January 1991--April 1991

Description: This report describes a low-cost, mass-producible 10X concentrator system that has been claimed to produce electricity at $0.04/kWh. It details changes in manufacturing techniques that could produce a concentrator system at a selling price of $0.71/W. (A simple design and a minimum number of parts and manufacturing steps reduced production costs.) Present production techniques, changes to improve these techniques, impediments to changes, and solutions to the impediments are described. This 10X concentrator system uses available components and manufacturing processes and one-sun solar cells in conjunction with inexpensive plastic lenses to generate about eight times the amount of electricity normally produced by these cells.
Date: November 1, 1991
Creator: Kaminar, N.; McEntee, J. & Curchod, D.
Partner: UNT Libraries Government Documents Department

Co-validation of three methods for optical characterization of point-focus concentrators

Description: Three different methods for characterizing point-focus solar concentrator optical performance have been developed for specific applications. These methods include a laser ray trace technique called the Scanning Hartmann Optical Test, a video imaging process called the 2f Technique and actual on-sun testing in conjunction with optical computer modeling. Three concentrator test articles, each of a different design, were characterized using at least two of the methods and, in one case, all three. The results of these tests are compared in order to validate the methods. Excellent agreement is observed in the results, suggesting that the techniques provide consistent and accurate characterizations of solar concentrator optics.
Date: October 1, 1994
Creator: Wendelin, T. J. & Grossman, J. W.
Partner: UNT Libraries Government Documents Department

Highlights of recent balance of system research and evaluation

Description: The cost of most photovoltaic (PV) systems is more a function of the balance of system (BOS) components than the collectors. The exception to this rule is the grid-tied system whose cost is related more directly to the collectors, and secondarily to the inverter/controls. In fact, recent procurements throughout the country document that collector costs for roof-mounted, utility-tied systems (Russell, PV Systems Workshop, 7/94) represent 60% to 70% of the system cost. This contrasts with the current market for packaged stand-alone all PV or PV-hybrid systems where collectors represent only 25% to 35% of the total. Not only are the BOS components the cost drivers in the current cost-effective PV system market place, they are also the least reliable components. This paper discusses the impact that BOS issues have on component performance, system performance, and system cost and reliability. We will also look at recent recommended changes in system design based upon performance evaluations of fielded PV systems.
Date: October 1, 1994
Creator: Thomas, M. G. & Stevens, J. W.
Partner: UNT Libraries Government Documents Department

Testing of the SAIC facets on the stretched-membrane dish

Description: A dish structure sized for a 25 kW electric dish/generator system has been installed at the National Solar Thermal Test Facility (NSTTF). The dish is used as an optical test bed for prototype stretched-membrane facets. Sandia procured twelve (12) stretched-membrane facets from Science Applications International Corporation (SAIC) for installation on the dish structure. Individual facet optical performance was measured using the 2f test method under development at Sandia. Four facets were tested on-sun using the test truss at the NSTTF prior to installation on the dish structure. During the final stages of the structure assembly, the facets were installed for testing. A distant light source alignment technique was used to align the facets to a 9 meter focal length. The alignment and focus were optimized with on-sun measurements of individual facet characteristics. On-sun flux maps of the image were made using the Beam Characterization System. Calorimetry tests were performed using a cold water calorimeter. This data was used to create a power intercept curve and predict the power inside apertures of different sizes. The focal plane was moved to 9.95 meters and the alignment, optimization, and on-sun tests repeated. This paper presents the results of the testing on the individual facets as well as the results of testing the dish system. The effects of the change in focal length on the optical performance of the dish are shown in the results.
Date: April 1, 1994
Creator: Grossman, J. W.; Erdman, W. W.; Houser, R. M. & Davenport, R.
Partner: UNT Libraries Government Documents Department

CIRCE2/DEKGEN2: A software package for facilitated optical analysis of 3-D distributed solar energy concentrators. Theory and user manual

Description: CIRCE2 is a computer code for modeling the optical performance of three-dimensional dish-type solar energy concentrators. Statistical methods are used to evaluate the directional distribution of reflected rays from any given point on the concentrator. Given concentrator and receiver geometries, sunshape (angular distribution of incident rays from the sun), and concentrator imperfections such as surface roughness and random deviation in slope, the code predicts the flux distribution and total power incident upon the target. Great freedom exists in the variety of concentrator and receiver configurations that can be modeled. Additionally, provisions for shading and receiver aperturing are included.- DEKGEN2 is a preprocessor designed to facilitate input of geometry, error distributions, and sun models. This manual describes the optical model, user inputs, code outputs, and operation of the software package. A user tutorial is included in which several collectors are built and analyzed in step-by-step examples.
Date: March 1, 1994
Creator: Romero, V. J.
Partner: UNT Libraries Government Documents Department

Uniform flux dish concentrators for photovoltaic application

Description: Researchers at the National Renewable Energy Laboratory (NREL) have designed a unique and innovative molded dish concentrator capable of producing a uniform flux profile on a flat target plane. Concentration levels of 100--200 suns, which are uniform over an area of several square inches, can be directly achieved for collection apertures of a reasonable size ({approximately}1.5-m diameter). Such performance would be immediately applicable to photovoltaic (PV) use. Economic concerns have shown that the proposed approach would be less expensive thatn Fresnel lens concepts or other dish concentrator designs that require complicated and costly receivers to mix the flux to obtain a uniform distribution. 12 refs.
Date: May 1, 1992
Creator: Jorgensen, G. & Wendelin, T.
Partner: UNT Libraries Government Documents Department

Membrane dish analysis: A summary of structural and optical analysis capabilities

Description: Research at SERI within the Department of Energy`s Solar Thermal Technology Program has focused on the development of membrane dish concentrators for space and terrestrial power applications. As potentially lightweight, inexpensive, high-performance structures, they are excellent candidates for space-deployable energy sources as well as cost-effective terrestrial energy concepts. A thorough engineering research treatment of these types of structures consists primarily of two parts: (1) structural mechanics of the membrane and ring support and (2) analysis and characterization of the concentrator optical performance. It is important to understand the effects of the membrane`s structure and support system on the optical performance of the concentrator. This requires an interface between appropriate structural and optical models. Until recently, such models and the required interface have not existed. This report documents research that has been conducted at SERI in this area. It is a compilation of several papers describing structural models of membrane dish structures and optical models used to predict dish concentrator optical and thermal performance. The structural models were developed under SERI subcontract by Dr. Steele and Dr. Balch of Stanford University. The optical model was developed in-house by SERI staff. In addition, the interface between the models is described. It allows easy and thorough characterization of membrane dish systems from the mechanics to the resulting optical performance. The models described herein have been and continue to be extremely useful to SERI, industry, and universities involved with the modeling and analysis of lightweight membrane concentrators for solar thermal applications.
Date: November 1, 1991
Creator: Steele, C. R.; Balch, C. D.; Jorgensen, G. J.; Wendelin, T. & Lewandowski, A.
Partner: UNT Libraries Government Documents Department

Outdoor testing of advanced optical materials for solar thermal electric applications

Description: The development of low-cost, durable advanced optical materials is an important element in making solar energy viable for electricity production. It is important to determine the expected lifetime of candidate reflector materials in real-world service conditions. The demonstration of the optical durability of such materials in outdoor environments is critical to the successful commercialization of solar thermal electric technologies. For many years optical performance data have been collected and analyzed by the National Renewable Energy Laboratory (NREL) for candidate reflector materials subjected to simulated outdoor exposure conditions. Much of this testing is accelerated in order to predict service durability. Some outdoor testing has occurred but not in a systematic manner. To date, simulated/accelerated testing has been limited correlation with actual outdoor exposure testing. Such a correlation is desirable to provide confidence in lifetime predictions based upon accelerated weathering methods. To obtain outdoor exposure data for realistic environments and to establish a data base for correlating simulated/accelerated outdoor exposure data with actual outdoor exposure data, the development of an expanded outdoor testing program has recently been initiated by NREL. Several outdoor test sites will be selected based on the solar climate, potential for solar energy utilization by industry, and cost of installation. Test results are site dependent because exposure conditions vary with geographical location. The importance of this program to optical materials development is outlined, and the process used to determine and establish the outdoor test sites is described. Candidate material identification and selection is also discussed. 10 refs.
Date: May 1992
Creator: Wendelin, T. J.; Jorgensen, G. & Goggin, R. M.
Partner: UNT Libraries Government Documents Department

Modification of flux profiles using a faceted concentrator

Description: The use of a faceted solar concentrator allows for some flexibility in aiming strategy and in the intensity of the resulting flux profile at the target. This can be an advantage when considering applications that do not necessarily require maximum concentration, particularly emerging, new applications in solar processed advanced materials. This paper will describe both an analysis of predicted flux profiles for several different aiming strategies using the SOLFUR computer code and experiments to characterize the actual flux profiles realized with a selected aiming strategy. The SOLFUR code models each of the furnace components explicitly. Aim points for each facet can be specified. Thus many strategies for adjusting aim points can be easily explored. One strategy calls for creating as uniform a flux over as large an area as possible. We explored this strategy analytically and experimentally. The experimental data consist of flux maps generated by a video imaging system calibrated against absolute flux measurements taken with circular foil calorimeters. Results from the analytical study and a comparison with the experimental data indicate that uniform profiles can be produced over fairly large areas.
Date: January 1, 1993
Creator: Lewandowski, A.; Scholl, K. & Bingham, C.
Partner: UNT Libraries Government Documents Department

DOE Solar Process Heat Program: FY1991 Solar Process Heat Prefeasibility Studies activity

Description: During fiscal year (FY) 1991, the US Department of Energy (DOE) Solar Process Heat Program implemented a Solar Process Heat Prefeasibility Studies activity. For Program purposes, a prefeasibility study is an engineering assessment that investigates the technical and economic feasibility of a solar system for a specific application for a specific end-user. The study includes an assessment of institutional issues (e.g., financing, availability of insurance, etc.) that impact the feasibility of the proposed solar project. Solar process heat technology covers solar thermal energy systems (utilizing flat plate or concentrating solar Collectors) for water heating, water preheating, cooling/refrigeration, steam generation, ventilation air heating/preheating, etc. for applications in industry, commerce, and government. The studies are selected for funding through a competitive solicitation. For FY 1991, six projects were selected for funding. As of August 31, 1992, three teams had completed their studies. This paper describes the prefeasibility studies activity, presents the results from the study performed by United Solar Technologies, and summarizes the conclusions from the studies that have been completed to date and their implications for the Solar Process Heat Program.
Date: November 1, 1992
Creator: Hewett, R.
Partner: UNT Libraries Government Documents Department

Measurement of the radiative transport properties of reticulated alumina foams

Description: This paper presents a method for determining radiative transport properties of reticulated materials. The method has both experimental and analytical components. A polar nephelometer is used to measure the scattering profile of a sample of the reticulated material. The results of a Monte Carlo simulation of the experiment are then combined with the experimental results to give the scatter albedo and extinction coefficient. This paper presents the results of using this method to determine the radiative transport properties of four different porosities (10, 20, 30, 65 pores per inch) of cylindrical reticulated alumina samples ranging in thickness form 0.5 inches to 2. 5 inches.
Date: December 1, 1992
Creator: Hale, M. J. & Bohn, M. S.
Partner: UNT Libraries Government Documents Department